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I have had the best results with paste flux and 45% silver solder.Make sure your copper and steel are real clean then get your tubing warm with your torch and apply your flux.I never had good luck applying the flux to cold metal.Keep most of your heat on the copper when it starts to flow move your heat towards the steel this will draw the solder up into the joint. Watch out and don't let the steel get cherry red.

Practice on scrap.

Originally posted by josan6 ...brazing a steel to copper connection...

Silphos contains phosphorus.
Phosphorus is not intended for use with steel.
It might stick to steel, but it causes the steel to become brittle. It's a leak, waiting to happen.
Use 45% instead, with the proper paste flux.

Clean everything.
Keep excess flux out of the system.
Coat the tube & fitting with flux, then slip it together.
(Part of the job of the flux is to protect against oxidation while heating.)
Watch the flux. It will indicate temperature.
At 212, the water boils out and the flux becomes dry. Keep heating, slowly.
When the proper temperature has been reached, the flux will turn clear & quiet.
Don't overheat.
Practice, practice, practice, on some scrap.

(Sorry, R12rules, I did a search & many, many unrelated threads showed up.
Including, "Who wants to argue with me?".
I thought I might help Josan6 narrow it down a little.)

Originally posted by fixerup I have had the best results with paste flux and 45% silver solder.Make sure your copper and steel are real clean then get your tubing warm with your torch and apply your flux.I never had good luck applying the flux to cold metal.Keep most of your heat on the copper when it starts to flow move your heat towards the steel this will draw the solder up into the joint. Watch out and don't let the steel get cherry red.

Thats the real trick to it i think.

thehumid1-------I live in NJ, a state where it's free to come in but you have to pay to leave!